Implantable cardiac therapy devices (ICTDs) enjoy widespread use for providing convenient, portable, sustained therapy for cardiac patients with a variety of cardiac arrhythmias. ICTDs may combine a pacemaker and defibrillator in a single implantable device. Such devices may be configured to provide ongoing cardiac pacing in order to maintain an appropriate cardiac rhythm. In addition, should the ICTD detect that the patient is experiencing an episode of ventricular fibrillation (or an episode of ventricular tachycardia), the ICTD can deliver appropriate defibrillation therapy.
Cardiac rhythm management (CRM) therapies require not only an ICTD, but also the placement of electrical leads threaded through blood vessels and typically into the heart itself. Patients with implanted electrical leads benefit from leads which exhibit optimized properties in terms of size (that is, minimal lead width or diameter), flexibility, strength, and reliability (including resistance to breaking), and various electrical properties such as low impedance (in order to carry large current loads).
With advances in both CRM therapy and ICTD technologies, the device implant pathway can become busy with three or more cables (for example, cables may be required for treating bradycardia, tachycardia, defibrillation, cardiac pacing, for standalone sensors, etc.). These multiple leads may need to be placed inside only one or two veins, which in turn benefit from smaller size leads to ensure adequate circulation through the blood vessels. Adding new sensor based diagnostic features, such as LAP (left atrial pressure), RVP (right ventricular pressure), and SvO2 (blood oxygen sensor), requires creating additional space in the implant pathway or the lead body for the diagnostic circuits. Therefore, the addition of such sensors requires that the regular ICD lead diameter again must be reduced. Potential target drug delivery and target biological therapy delivery of tissues, cells, antibodies genes, etc. needs to be specifically delivered via a lead channel in the given vein with the new ICD leads. All of these therapeutic demands create requirements for the thinnest possible leads consistent with other lead requirements (flexibility, durability, low electrical resistance, and others).
With recent advances in cardiac therapies, alternative ICD lead implant sites are increasingly used. These include: the right ventricular outflow tract (ROT), the right ventricular (RV) high septum, and other sites in the right heart; and also the cardiac septum (CS), the great cardiac vein, and other areas of the left heart. To this end, the ICD leads must be robust and flexible for site specific positioning, and for ease of implantation through the torturous and complex implant pathways. ICTD leads also require improved acute and chronic stability at the desired site to reliably deliver the desired therapies for the entire design life of the system.
As is well known in the art, there are also different delivery methods to implant leads in the heart. The ICD lead should be compatible with traditional stylet delivery, and also be compatible with the emerging screw-driver stylet and/or slitable/steerable catheter, which benefits even more from a smaller size ICD lead.
Yet another aspect of lead design is enhanced lead removability, which becomes possible with leads that exhibit only minor fibrotic encapsulation. The degree of fibrosis engendered by a lead may be altered by optimized lead body materials and coatings, but here again a reduced electrical lead size contributes as well.
Yet another objective of lead design is MRI compatibility, which places specific requirements on the conductors for sizes, layout, insulation, etc.
The various operational requirements for ICTD leads, as specified above, create competing design requirements. In general, thinner leads contribute to flexibility and allow for maximum circulation within blood vessels. At the same time, it is known that fretting fatigue is the primary failure mode of a small-sized lead made of multiple filament wires; for example, the center filament wire is usually broken first in an existing 1×19 cable where all of the wires are of the same size. Further, smaller leads exhibit lower tensile strength. Also, when the lead size becomes smaller, the DC resistance of the cable increases, which in turn decreases the capability to carry large currents.
It will be noted that while implantable leads are essential in the field of cardiac rhythm management (CRM) therapies, they are employed in many other biomedical applications as well. For example, implantable leads have applications in neurology for treatment of Parkinson's disease, epilepsy, chronic back pain, and other conditions. Many of the requirements identified above, such as small size (i.e., being as thin as possible), flexibility, durability, and low resistance are requirements for these other applications as well.
What is needed, then, is an apparatus for an implantable lead for use with an ICTD, and for other implantable medical applications as well, with a smaller size lead which none-the-less exhibits optimized performance for implantation in relation to existing leads including, for example:                flexible bending but higher tension strength;        higher fatigue life;        stronger ability to resist kinking;        better electrical conductivity;        lower DC resistance to carry large current during cardiac shocking.        